Page 163 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

CHAPTER

6
Evaporators

6.1 Introduction
Evaporation is a common unit operation used in chemical, food, and pharmaceutical industries and
also in desalination, water treatment, and power plants. In the process, a solvent is boiled out of a
solution to concentrate a product. In most cases, the solvent is water, and the product is a concentrated
solution, or in some cases, it can be a slurry. The product can also be solid, as in the case of evaporative
crystallizers used for the production of salt and ammonium sulfate crystals from their solution.
Concentrating solutions like caustic soda lye, sugar syrup, fruit juice, paper mill effluents, etc., employ
evaporators.
Heat transfer to a solution or slurry distinguishes the evaporator from a dryer where heat is transferred
to a solid to vaporize the accompanying volatile liquid. The
vapor from an evaporator is not separated into components as in
the process of reboiling in the reboiler of a distillation column.
Evaporator, Dryer, and Reboiler
In the latter case, the purpose is to generate a vapor stream that is
returned back to the column for component-wise separation.
Although evaporators involve simultaneous heat and mass transfer, they are classified as heat
transfer equipment, as the process rate is governed by the rate of heat transfer. A substantial amount of
heat energy is consumed in the evaporation systems, possibly next to distillation.

6.2 Components of an evaporation system
The basic evaporator unit is comprised of the evaporator body (vessel), arrangements for supplying the
heat, and also the auxiliaries for pulling of vacuum, recirculation of liquid if required, and condensing
the spent vapors. Tubes have traditionally been used as heat exchange surface. The heat transfer
arrangement can be internal to the body or an external heat exchanger. When internal, the tube bundle,
called calandria, has a pair of tube sheets. The central tube in calandria has a larger diameter so that it
acts as “downtake,” leading to the circulation of liquid in the body. Rest of the tubes having a smaller
diameter act as “risers.” A typical short tube calandria type evaporator is shown in Fig. 6.1. Evaporators
with an external heating surface have the advantage that the heat transfer area is not limited by the size
of the evaporator body. Circulation through the external heater in most cases is by forced circulation, as
shown in Fig. 6.2. Plate type exchangers are also used as external heaters in recent designs. Circulation
of the liquid past the heating surface may be induced by boiling or by pumps. In the latter case, boiling

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